Method for making membrane faced acoustical tile



Oct. 27, 1964 D. A DEMKE ETAL METHOD FOR MAKING MEMBRANE FACED ACOUSTICAL TILE 2 Sheets-Sheet 1 Filed May 6, 1960 Inf/ftfor'g. pofiala/ f7- Demfie. fiorzald dpuray. & amt 6mm) Oct. 27, 1964 D. A DEMKE ETAL 3,154,453

METHOD FOR MAKING MEMBRANE FACED ACOUSTICAL TILE Filed May 6, 1960 2 Sheets-Sheet 2 United States Patent 3,154,453 NETHUD FOR MAKHNG MEMBRANE FACEI) ACUUSTEQAL TILE Donald A. Demhe, Western Springs, and Donald G. Duray, Carpentersviile, Ill, assignors to The Celoten (Jorporation, Chicago, Ill, a corporation of Delaware Filed May 6, I960, Ser. No. 27,311 6 Claims. (Cl. 156-2l2) This invention relates to a method for making acoustical tile and more particularly to a method of making acoustical tile having a thin, taut, vibratile membrane stretched over the surface of a sound absorbing body.

The vast majority of acoustical ceilings installed in commercial and residential buildings employs the type of acoustical tile which comprises a body of sound absorbing material, such as vegetable fiber or mineral wool fiber, having microscopic void openings accessible either directly from the surface or through fissures or perforations in its face so that incident sound waves may enter into the interstices of the sound absorbing material and be therein dissipated by friction. The individual tile may be coated with paint to preserve its surface, aid in flame retardency, and in some instances, add to the decorative effect of the tile.

Such commercial acoustical tile, now available, has serious disadvantages in actual use. The presence of the microscopic openings, perforations, or fissures in the face of the tile provides an area for germination and multiplication of microbes and other types of disease-bearing germs. In addition, dust and dirt, always present in a room, tend to accumulate in the perforations or fissures,

thus degrading the sound absorbing efiiciency of the tile.

The conventional tile is difficult to keep clean because it is difficult to penetrate the perforations or fissures with a cleaning solution. Additionally, the absorption of moisture in the tile during cleaning operations at times causes deterioration of the tile.

Because of these disadvantages there has been developed a type of acoustical tile in which a thin, taut, vibratile membrane is stretched across the surface of the sound absorbing body. The use of this membrane results in a tile which is eificient in sound absorbing characteristics, and yet is easy to clean and maintain in a sanitary condition. This tile is of particular importance in hospital operating rooms and in kitchens where it is necessary to maintain the tile free from germs or dirt. While this latter type of tile is available, its manufacture at the present time is difiicul-t and in general results from a very inefiicient hand operation.

In addition, presently available acoustical tiles having a tightly stretched membrane over the surface of a sound absorbing body are manufactured by using a heat-shrinking process in which the tile is secured to the edges of the sound absorbing body and then heat-shrunk to become a taut, vibratile membrane. One of the disadvantages of this process lies in the fact that for decorative purposes it is sometimes desirableto use a membrane which has an embossed surface. It is apparent that if the heat-shrinking principle is applied to the embossed membrane, the embossing will immediately disappear.

It is, therefore, an object of the present invention to provide an acoustical tile having a thin, taut, vibratile membrane stretched over the surface of a sound absorbing body, which tile may be easily cleaned to provide a healthful and almost sterile condition.

It is a further object of the present invention to provide an acoustical tile having a taut membrane stretched over the surface of a sound absorbing body, which tile may be easily and efficiently manufactured in semiautomatic or automatic production.

BJMASB Patented Get. 2?, 1964 lCe It is yet another object of the present invention to provide an acoustical tile having a tightly stretched membrane which is pleasing in appearance and may have an embossed surface.

In general, the present invention is directed to a process for producing in a semi-automatic or automatic manner an acoustical tile having a thin, taut, vibratile membrane stretched over the surface of a sound absorbing body and secured to the sides of the body. The process comprises providing a heat-shielding member, stretching a thin membrane below the heat-shielding member, placing a preformed body of sound absorbing material below the membrane and in registration with the shield, moving the shield or body relative one to another so as to form a tight sandwich enclosing the membrane, applying heat and vacuum to the membrane so as to deform the membrane around the edge of the sound absorbing body while yet preventing heat from reaching the portion of the membrane covering the central surface of the body, releasing the heat and vacuum, cooling the now secured membrane, and subsequently trimming the excess membrane from the tile.

Other objects will become apparent when the invention is considered in greater detail, as shown in the accompanying drawings, in which like numerals indicate like elements and in which:

FIGURE 1 is a perspective view of the finished tile with a portion broken away to illustrate its internal structure;

FIGURE 2 is an end elevation of a series of acoustical tiles during various steps of the process;

FIGURE 3 is a top plan view of the apparatus used in manufacturing the tile;

FIGURE 4- is a cross-sectional elevational view of the apparatus of FIGURE 3 taken along lines 4-4 thereof;

FIGURE 5 is a top view of a portion of the apparatus of FIGURE 3 taken along lines 5-5 thereof; and

FIGURE 6 is a diagrammatic view of a heat treating apparatus used in one of the steps of manufacture of the tile of the invention.

The nature of the present invention may be best understood with reference to the drawings and more particularly to FIGURE 1, wherein the acoustical tile 10 is illustrated as comprising a body 11 having a thin, taut, vibratile member 12 secured thereto along the upper portion of sides 14 of the body 11 by a suitable adhesive 16. Membrane 12 is maintained in a taut condition against the upper surface of body I1 without being secured thereto, and is free to vibrate so as to aid in the absorption of sound energy impinging on the acoustical tile It To make the finished acoustical tile more attractive, the upper edges of sides 14 are beveled, and the adhesive 16 is placed in the form of a thin band just below the lower edge of the bevel. As will be discussed in detail later, the excess membrane extending below the adhesive is trimmed away by suitable, but conventional trimming apparatus.

The body portion 11 of the tile Iii comprises a rigid, light weight, mineral wool tile formed of loose mineral Wool and binder and formed on a modified Fourdrinier machine. A suitable body may be, for example, the sound absorbing acoustical tile manufactured by The Celotex Corporation of Chicago, Illinois, and sold under the trademark Mufiletone. However, it should be understood that as long as the body of the tile has efiicient sound absorbing characteristics and is rigid enough to be mounted without sagging or warping, any type of conventional acoustical material may be used. Thus, the .body portion 11 may be of vegetable fiber or glass fiber utilizing a suitable binder for rigidity and having a porous surface for efficient sound absorption.

The body portion 11 is preferably formed in the shape of a rectangular block, 12" x 12" x 11/16", to conform to the present conventional acoustical practice, although other sizes of body 11 may be used if desired.

In order to further prepare the basic core or body 11 for kerfing or machining after the application of the membrane 12, the sides 14 and the back of body 11 are sized with a conventional glue size which is then dried by radiant heaters. Additionally, the glue size further adds to the rigidity of the acoustical tile to minimize sagging or warping.

Membrane 12 is a vinyl film with an embossed pattern. More specifically, membrane 12 is a rigid, unplasticized embossed vinyl film having a thickness of .002 inch. The unplasticized film is superior to plasticized film in that it has greater dimensional stability, less tendency toward film distortion or yellowing because of lack of plasticizer migration, and greater strength and durability. The use of an embossed surface has proved advantageous in that it conceals slight surface defects in the tile body which would otherwise be apparent through unembossed film.

The adhesive 16 can be any suitable adhesive, although a formulated synthetic rubber latex such as that commercially available as Resyn 76-4454, manufactured by the National Starch and Chemical Company of Plainfield, New Jersey, was found to be particularly desirable.

The operation of the acoustical tile in absorbing noise or unwanted sound energy occurs as a result of the interaction of :the vibratile vinyl membrane 12 and the sound absorbing body 11. Sound impinging upon the taut membrane 11 sets it in motion. A small amount of the sound energy is absorbed by the inertia of the vinyl membrane. Most of the sound energy is transmitted by the membrane and is absorbed in the porous fibrous mass of the body 11.

To describe the manner of making the acoustical tile of FIGURE 1, reference is now made to FIGURES 2-6. FIGURE 2 shows the tile being formed at two different steps in the process. At the right hand side of FIGURE 2 there are shown two adjacent acoustical bodies 11 with the vinyl film 12 drawn tightly across their surfaces. At the left hand side of FIGURE 2 there are shown two adjacent acoustical bodies 11, with the film 12 drawn tightly across their surfaces and down over the adhesive band 16, prior to their being separated. Details of these two steps will be discussed hereinafter following a discussion of the apparatus used in the process as shown in FIG- URES 3-5.

The apparatus which forms and secures the vinyl membrane 12 tightly over the top surface of body 11 comprises, in general, a vacuum chamber 20, a frame and heat mask assembly 21, a heating unit 22, and a cooling arrangement 24. In addition support means rotatably support a roll of vinyl film or membrane 12.

The acoustical tile manufacturing apparatus of the invention, illustrated in FIGURES 3 to 5, rests upon a foundation 27 and is supported thereon by upright frame members 29. A vacuum chamber 20, which is in the form of a rectangular box having a bottom wall 30 and side walls 32 as well as a front wall (not shown) and a back wall 34, is mounted between frame members 29. A pair of pillars 36 are mounted on respective air cylinders 38 for raising and lowering the vacuum chamber 20. Air cylinders 38 are operated through intake and outflow pipes 40 and are connected to a suitable pneumatic pump (not shown) for actuation. The operation of the air cylinders 38 in raising and lowering the vacuum chamber 20 is conventional and such details have been omitted.

For purposes of creating a vacuum therein, chamber 20 is connected to an air suction pump 42 driven by a motor 44 via a belt drive 46. Suction pump 42 and motor 44 are also mounted on foundation 27. Pump 42 is connected to chamber 20 by means of a pipe 47 mounted slidably within a depending collar 48 which surrounds an opening 49 in the bottom wall 30 of chamber 20. The

slidable connection between pipe 47 and collar 48 is sealed in a conventional manner so as to be air tight.

In addition, chamber 20 has flanges 51 extending outwardly from each of its four sides. A block 53 is secured on the top of flange 51 around all four sides of the chamber 20 to serve as the lower member of a clamping mechanism which holds the vinyl film 12 tightly across the surface of the body 11 prior to the suction step.

Enclosing the top of suction chamber 20, is a foraminous tray 55 upon which the tile bodies 11 are placed. Tray 55, which is preferably of rigid perforated metal, is mounted upon support bars 57 which are connected parallel to the bottom wall 30 and slightly below the upper edges of sides 32 of chamber 20. Tray 55 is removable from its illustrated position.

In order to facilitate entry and removal of tray 55 from its illustrated position, the apparatus is provided with an input conveyor 59 and an output conveyor 60. Conveyors 59 and 60 comprise a conventional frame arrangement 62 having a plurality of rolls 64 mounted on shafts 65. In operation, fifteen acoustical bodies 11 are placed on the foraminous tray 55 to form a rectangle of three bodies by five bodies. The bodies are separated slightly from adjacent ones on the tray. In practice, each body 11 is separated by a distance of about one to one and one-half inches from adjacent bodies. After the assembly of bodies on the tray 55, the tray is then inserted, utilizing conveyor 59, into the position illustrated in FIGURE 4. After completion of the operation, the tray 55 is removed over conveyor 60.

A framework 67, to which heat shields 69 are attached, is mounted above the vacuum chamber 20 and connected to the main support members 29. Framework 67 comprises a plurality of bars or rods 71 which are connected in spaced parallel relationship to a rectangular frame 73. The heat shields 69 are mounted in a rectangular grid of 3X5 heat shields and are connected by welding or rivets in a conventional manner to respective pairs of the rods 71 as shown.

Each of the heat shields 69 is constructed of sheet metal in the form of a hollow rectangular solid having an open bottom. The open area of the rectangular solid is the same as the upper surface of its corresponding acoustical body 11. Inserted in the hollow rectangular solid and extending slightly outwardly from the open bottom thereof is a rubber filler 75. Rubber filler 75 is designed to tightly compress the membrane 12 against the upper surface of its associated acoustical bodies 11 and prevent any displacement of the vinyl sheet during the heat cycle step, which will be referred to hereinafter. To stiffen the heat shield 69 and prevent warping thereof, the rubber filler 75 is backed up by a sheet of hardboard 76.

Originally, it was thought that merely providing a metal heat shield to prevent the application of heat to the embossed surface of the vinyl sheet would be sufficient. However, it was discovered that such an arrangement was not suitable, and it was unexpectedly found that it was necessary to provide a rubber or resilient filler within the heat shield to make the tile without destroying the embossing. The rubber filler 75 also compensates for slight warping of heat shield 69 and applies equal pressure across the surface of the body 11.

Heating unit 22 comprises a metallic reflector 79 of rectangular area which is large enough to cover the entire area of the vinyl sheet 12 covering vacuum chamber 20. Reflector 79 has depending sides 80 to which are mounted wheels 82 on axles 83. U-shaped channel members 85 are mounted at each end of the apparatus to provide a track upon which wheels 82 can run. In operation, the heating unit is pushed rearwardly out of the way during the time that the vinyl of the membrane is pulled over the surface of the acoustical bodies 11. Subsequently, during the heat treatment step, the heating unit 22 is pulled forwardly so that it overlies the heat shields and the membrane 12 which extends between the heat shields 69. Infra-red lamps 36 are used to provide the heat necessary to soften the membrane during the heat processing step. Lamps 86 are connected to a conventional source of electrical power (not shown) for their operation. The lower portion 88 of channel members 85 are reinforced to provide a clamping means which cooperates with clamping means 53 connected to the suction box to firmly hold the vinyl film 12 in place during the softening step.

A cooling arrangement 24 comprising a pair of fans 89 is mounted above the heat mask assembly 21 on frame members 91. Frame members 91 are connected to support members 29 by means of depending frames 93. Fans 39 are connected to a suitable source of electrical power (not shown) for their operation.

The vinyl membrane 12, in the form of a roll 94, is rotatably mounted on extensions 95 of support members 91. A shaft 97 is inserted through the roll 94 and is journaled for rotation on bearings @S which are supported on extensions 95.

The operation of the apparatus (FIGURES 3-5) used in the application of the vinyl membrane 12 to the acoustical bodies 11 will now be described.

The bodies 11 are prepared in the manner previously described by glue-sizing their backs and edges and applying thereto bands of adhesive 16. In addition it was discovered that during the vacuum forming step (discussed in greater detail hereinafter) there was a partial compression of the basic body which resulted in loosening of the film on the board surface, resulting in sag when the tile is installed face down in a ceiling support system. In order to insure against this subsequent sag problem, it was determined that the surface hardness of the board should be in excess of .075 as measured by the penetration of a %1 steel ball under a five pound load. As long as the surface hardness equaled or exceeded this minimum value, the acoustical tile as produced was not subject to resulting sag of the vinyl membrane caused by board compression.

Bodies 11 are placed on tray and aligned so that upon insertion of the tray into the apparatus, each of the bodies Ill lies below and in registration with a corresponding one of the heat masks 69.

Adjacent ones of the bodies II are separated by a distance of about one-inch to one and one-fourth inch between edges.

After the assembly of the bodies 11 on the tray, the tray is then rolled across conveyor 59 and inserted into vacuum chamber 26 so that it rests upon supports 57. At this time it should be realized that the vacuum chamber 2t) is lowered by means of pistons 36 so that the upper flanges thereof and the clamping means 53 mounted thereon lie in the extension of the plane of conveyor 59. Vinyl membrane 12 is then pulled from the roll 94 across the upper surface of bodies 11 and between the clamping means 53 and 855 at each end of the apparatus. Air cylinders 38 are then actuated to raise vacuum chamber 20 by means of cylinders 36 until the clamping means 53 and 83 engage to tightly hold the vinyl membrane 12 across the upper surfaces of bodies ill. At this time the rubber fillers '75 of the heat masks 69 are tightly compressed against the vinyl membrane 12 and against the upper surfaces of bodies 11.

Heating unit 22, which has been preheated to a temperature of about 1100" F., is rolled forward over the heat masks 69 and vinyl membrane 12. The heating unit 22 is held in its operative position for about three to four seconds. Simultaneously, a vacuum of about 10 inches of mercury is applied to the basic board through vacuum chamber 20 and pump unit 42. The conjoint operation of the heat applied by heating unit 22 and the vacuum applied by vacuum box 26 causes the vinyl membrane 12 to be drawn tightly across the surface of the bodies 11 and downwardly across their sides.

Details of the relationship between the vinyl membrane 12 and the bodies 11 during these two steps may be more easily seen with reference to FIGURE 2. The right hand portion of FIGURE 2 illustrates the membrane being held tightly across the upper surface of bodies 11, while the left hand illustration of FIGURE 2 shows the vinyl membrane after the heat and suction steps have been simultaneously applied. It can be seen that the vinyl in the left hand illustration of FIGURE 2 is pulled downwardly and stretched tightly against the sides 14 of adjacent bodies Ill. The adhesive 16, at this point, tightly bonds the vinyl membrane 12 to the tile bodies 11.

While the vacuum is still applied, the heating unit 22 is rolled back away from its operative position and the vinyl membrane 12 is allowed to cool for about five to ten seconds. The fans 89 are utilized to assist in this cooling step.

At the time the vacuum is removed, the vacuum box 2% is lowered to release the bodies 11 from heat masks 69, and tray 55 is then removed from the apparatus by utilization of conveyor 60.

Following the application of the vinyl film and its bonding to the individual bodies 11 to form acoustical tiles It the acoustical tiles are separated by cutting the webs of vinyl membrane 12 which connects the adjacent tiles. The tiles are further fabricated by removing the excess vinyl membrane 12 by conventional cutting and trimming devices (not shown).

The apparatus for forming the acoustical tile 10 and its method of operation have been described. However, in the actual installation of the acoustical tile 10, as formed in the above-described manner, there were found to be certain difliculties. The vinyl film 12 has an apparent tendency to expand and contract with the rise and fall of the temperature in the room on the ceiling of which the acoustical tile 10 had been mounted. The acoustical tile showed a wrinkling effect when the vinyl film expanded. Further experimentation was undertaken to determine the cause and solution of this problem. It was determined that it was necessary to treat the membrane 12 in such a manner that it would not be adversely affected by a rise in temperature of up to about 150 F. in the room where it was applied.

it was determined that an additional heat treating step was necessary to overcome the tendencies of the vinyl membrane 12 to sag in actual use. This step comprises heat treating the acoustical tile It} after it has been formed, and such step may be better explained with reference to FIGURE 6.

FIGURE 6 illustrates a heat chamber 165 which is heated by means of flame from a gas jet 1%. It should be realized that heat chamber can be any conventional apparatus which provides air heated to a temperature of about F. Heat chamber 105 is connected to an oven over a pipe lit). The pposite side of oven 10% is connected to a suction fan 112 through a pipe 113. In operation, the temperature of the oven is maintained at a constant temperature by means of the fiow of air from heat chamber N5 caused by the operation of suction fan 112.

After the acoustical tile It has been formed in the manner hereinbefore described, it is then placed in the oven 1% in order to relieve stresses which are formed in the vinyl membrane 12 during its manufacture. The following table indicates the relationship between the temperature maintained in the oven and the time period during which the tile It) remains in the oven:

Minutes 150" 5 2 /2 1%. .75

After the tile has been heat-tempered, it is then ready for commercial use.

While the exact nature of the heat treatment is not fully known or understood .by the applicant, the applicant believes that the following theoretical explanation is applicable.

The vinyl membrane 12 is made from polyvinyl chloride and is drawn to a thickness of .002 inch. During the original film manufacturing process, the polyvinyl chloride is heated, calendered and drawn. It is thought that in addition to the normal molecular stresses in the polyvinyl chloride film, additional directional stresses are introduced into the material during the stretching or drawing phase of the manufacturing process. In addition to the additional stresses put in the vinyl film during the manufacturing process, further stresses are introduced in the film during its application to the acoustical body 11. It is believed that while the vinyl membrane 12 is heat-tempered in the oven 108, it first relaxes to relieve the additional directional stresses, and then returns to the original tension which it had after being formed on the tile bodies 11. There is no additional tension placed in the film by the heat treating step over that initially established in the film by the vacuum and heat-forming processes.

As a result of this heat treatment step, the film 12 is better able to withstand temperature changes in the room in which it is mounted. It should be realized that because the film has an embossed surface, the maximum practical temperature to which it can be exposed in the oven is about 185". Above this temperature the embossing pattern disappears because of the softening of the vinyl which causes the embossing pattern to disappear. The lower limit of the temperature to which the acoustical tile is exposed is determined by the fact that at temperatures below 150 the time period for which exposure in the oven is required is lengthened beyond a commercially acceptable period for the efficient and economical fabrication of the tile.

While there has been set forth a specific example of the heat treatment as applied to the vinyl membrane 12, it should be realized that variations in the process for manufacturing the vinyl film will result in slight variations in the character of the directional stresses induced therein. Therefore, the time-temperature conditions under which the heat treatment step is carried on should be slightly varied depending upon the nature of the directional stresses induced in the film. Specifically, the temperature at which the vinyl film is calendered or stretched in its manufacture will vary somewhat from manufacturer to manufacturer, or even from batch to batch by the same manufacturer. Thus, the directional stresses may be relieved at different temperatures and for different times during the heat treatment step. However, these variations in the time-temperature characteristics during the heat treatment step are easily determinable by those skilled in the art and do not require extensive experimentation when considered in the light of the teachings set forth herein.

In summary, there has been set forth a method of producing an acoustical tile having a thin, taut, vibratile membrane secured across the surface of the sound absorbing body, and further steps in the process for relieving directional stresses in the vinyl membrane.

While there has been shown and described a particular embodiment of the invention, it will be obvious to those skilled in the art that various changes, modifications, and variations may be made therein without departing from the scope of the invention, and, therefore, it is intended to cover in the appended claims all such changes, modifications and variations as may fall within the true spirit and scope of the invention.

We claim:

1. The method of making an acoustical tile having a thin, taut, vibratile membrane secured in place relative to one surface of the body of said tile, said .body having at least one planar surface area and sidewalls depending from said surface, comprising the steps of applying an adhesive to said sidewalls, placing said membrane under tension in a plane above and in contact with one surface of said body, applying suction to said membrane to secure it tightly against the said surface of said body, shielding that portion of said membrane in contact with said planar surface of said body to prevent application of heat to said portion of said membrane, applying heat to the unshielded portion of said membrane simultaneously with the application of said suction for causing the unshielded portion of said membrane to flow and conform to said sidewalls, removing suction from said body and said membrane after the application of said heat, cooling said combined membrane and body to ambient temperature, reapplying heat to said membrane to relieve stresses therein and cooling said acoustical tile.

2 The method of making an acoustical tile having a thin, taut, vibratile membrane secured in place relative to one surface of the body of said tile, said body having t least one planar surface area and sidewalls depending from said surface, comprising the steps of applying an adhesive to said sidewalls, placing said membrane under tension in a plane above one surface of said body, raising said body against said membrane, applying suction to said membrane to secure it tightly against the said surface of said body, shielding that portion of said membrane in contact with said planar surface of said body to prevent application of heat to said portion of said membrane, applying heat to the unshielded portion of said membrane simultaneously with the application of said suction for causing the unshielded portion of said membrane to How and conform to said sidewalls, removing suction from said .body and said membrane after the application of said heat, cooling said combined membrane and body to ambient temperature, reapplying heat to said membrane to relieve stresses therein and cooling said acoustical tile.

3. The method of making a beveled acoustical tile having a thin, vibratile membrane, secured in place relative to one surface of the body of said tile, said body having at least one planar surface area, a beveled edge surrounding said surface area and sidewalls depending from said beveled edge, said finished tile having an embossed pattern over that area enclosed by said beveled edge, comprising the steps of applying an adhesive to said sidewalls, placing said membrane under tension in a plane above one surface of said body, raising said body against said membrane, applying suction to said membrane to secure it tightly against the said surface of said body, shielding that portion of said membrane in contact with said planar surface of said body to prevent application of heat to said portion of said membrane, applying heat to the unshielded portion of said membrane simultaneously with the application of said suction for causing the unshielded portion of said membrane to flow and conform to said sidewalls, removing suction from said body and said membrane after the application of said heat, cooling said combined membrane and body to ambient temperature, reapplying heat to said membrane to relieve stresses therein and cooling said acoustical tile.

4. The method of making an acoustical tile having a thin, taut, vibratile membrane secured in place relative to one surface of the body of said tile, said body having at least one planar surface area and sidewalls depending from said surface, comprising the steps of applying an adhesive to said sidewalls, placing said membrane under tension in a plane above and in contact with one surface of said body, applying suction to said membrane to secure it tightly against the said surface of said body, shielding that portion of said membrane in contact with said planar surface of said body to prevent application of heat to said portion of said membrane, applying heat to the unshielded portion of said membrane simultaneously with the application of said suction for causing the unshielded portion of said membrane to flow and conform to said sidewalls, removing suction from said body and said membrane after the application of said heat, cooling said combined membrane and body to ambient temperature,

reheating said membrane to a temperature not to exceed 185 F. to relieve stresses therein and subsequently cooling said acoustical tile.

5. The method of making an acoustical tile having a thin, taut, vibratile membrane secured in place relative to one surface of the body of said tile, said body having at least one planar surface area and sidewalls depending from said surface, comprising the steps of applying an adhesive to said sidewalls, placing said membrane under tension in a plane above and in contact with one surface of said body, applying suction to said membrane to secure it tightly against the said surface of said body, shielding that portion of said membrane in contact with said planar surface of said body to prevent application of heat to said portion of said membrane, applying heat to the unshielded portion of said membrane simultaneously with the application of said suction for causing the unshielded portion of said membrane to flow and conform to said sidewalls, removing suction from said body and said membrane after the application of said heat, cooling said combined 20 10 membrane and body to ambient temperature, reheating said membrane to a temperature of between 150 and 180 F. for between five minutes and three-quarters of a minute to relieve stresses therein and subsequently cooling said acoustical tile.

6. The method of making an acoustical tile in accordance with the steps of claim 5 in which said membrane is embossed polyvinyl chloride.

References Cited in the file of this patent UNITED STATES PATENTS 2,802,764 Slayter et a1. Aug. 13, 1957 2,828,799 Harrison Apr. 1, 1958 2,927,409 Heyer Mar. 8, 1960 2,958,172 La Branche Nov. 1, 1960 2,989,827 Groth June 27, 1961 FOREIGN PATENTS 123,758 Australia June 4, 1945 

1. THE METHOD OF MAKING AN ACOUSTICAL TILE HAVING A THIN, TAUT, VIBRATILE MEMBRANE SECURED IN PLACE RELATIVE TO ONE SURFACE OF THE BODY OF SAID TILE, SAID BODY HAVING A LEAST ONE PLANAR SURFACE AREA AND SIDEALLS DEPENDING FROM SAID SURFACE, COMPRISING THE STEPS OF APPLYING AN ADHESIVE TO SAID SIDEWALLS, PLACING SAID MEMBRANE UNDER TENSION IN A PLANE ABOVE AND IN CONTACT WITH ONE SURFACE OF SAID BODY, APPLYING SUCTION TO SAID MEMBRANE TO SECURE IT TIGHLY AGAINST THE SAID SURFACE OF SAID BODY, SHIELDING THAT PORTION OF SAID MEMBRANE IN CONTACT WITH SAID PLANAR SURFACE OF SAID BODY TO PREVENT APPLICATION OF HEAT TO SAID PORTION OF SAID MEMBRANE, APPLYING HEAT TO THE UNSHIELDED PORTION OF SAID MEMBRANE SIMULTANEOUSLY WITH THE APPLICATION OF SAID SUCTION FOR CAUSING THE UNSHILDED PORTION OF SAID MEMBRANE TO FLOW AND CONFORM TO SAID SIDEWALLS, REMOVING SUCTION FROM SAID BODY AND SAID MEMBRANE AFTER THE APPLICATION OF SAID HEAT, COOLING SAID COMBINED MEMBRANE AND BODY TO AMBIENT TEMPERATURE, REAPPLYING HEAT TO SAID MEMBRANE TO RELIEVE STRESSES THEREIN AND COOLING SAID ACOUSTICAL TILE. 